Hydroxy ethyl methacrylate (HEMA) and vinyl pyrrolidone (VP) are monomers commonly used in the formulation of contact lens materials. Various formulations with or without modifiers are known having water contents ranging from as little as about 25% upwards to about 80% at equilibrium. It is also known that modifiers such as methyl methacrylate may be mixed with HEMA and VP. Such prior art materials exhibit an oxygen permeability directly proportional to the water content although a slight shift from material to material may occur because of monomer ratios and/or the amount and/or type of modifier present. For example, such materials will normally exhibit an oxygen permeability of about 6-9 Barrer units (BU) when the water content is in the range of 40%-45%; 10-14 BU when the water content is in the range of 50%-60%, and, 25-40 BU when the water content is in the range of 70%-90% of the contact lens material. The high water content soft contact lenses (60-90%) are known to have a number of disadvantages which include a rapid water evaporation rate; low structural strength; lower resistance to biological and/or chemical attack, poorer demensional stability and a propensity to discolor.
Recently, siloxane-containing materials which have good oxygen permeability have been suggested for contact lens use. However, such materials are known for their incompatibility with hydrophilic monomers such as HEMA and VP. Therefore, these materials have found applicability only in the hard contact lens field with polymers such as cellulose acetate butyrate (CAB) and polymethyl methacrylate (PMMA).
U.S. Pat. No. 4,152,508 issued May 1, 1979 discloses the group of acrylic siloxanes used in the present invention and teaches that they are useful for making hard contact lenses. Although HEMA and VP are identified as possible components of the hard lens compositions, their total combined content is limited to 20% or less of the total composition and other required components prevent the preparation of a soft contact lens material.